In current display panels, the scanning frequency is ever increasing to support high definition image frame rates, especially for 3D image visualization, where the frame rate is double compared to the equivalent 2D visualization. With this trend, power dissipation, which is also tied to the size of the displays, is a serious concern. As a result, scan driver design is becoming more and more challenging.
Charge sharing techniques have been developed, and are being implemented by major panel manufacturers. The charge sharing method reuses (recycles) part of the electrical charge accumulated in the activated, commonly buffered, channel or gate line being turned off to assist in charging the next channel or gate line to be activated. If properly implemented, the charge sharing may procure a significant energy saving.
U.S. Pat. No. 7,750,715 discloses a method for generating a first clock signal in a first signal path in response to a first input signal, and a second clock signal in a second signal path in response to a second input signal. The first and second lock signals assume first and second clock levels to transfer electrical charges from an ancillary charge storage component to one and to the other output, respectively, to reduce power dissipation when performing a multichannel scanning, as in an LCD display.
Implementation of a charge sharing function to reduce power dissipation is also disclosed in data sheets of commercial devices TPS 65191 and TPS 65193, as provided by Texas Instruments. The charge sharing function is limited to a sharing between complementary outputs, and availability of relatively high voltage zener diodes in the silicon fabrication technology is required.
U.S. Published Patent Application No. 2010/0109995 discloses a gate driving device used in an LCD display. The gate driving device includes a plurality of gate lines, with each gate line including a plurality of output stages, a couple of complementary switches and a control module. The gate driving device implements a charge sharing function to reduce power consumption. The approach is not applicable to GOA panels because of the relatively high voltage rails required with this LCD technology for correct driving of the gate lines. Even a hypothetical implementation of the disclosed circuits with high voltage MOSFETs would not work because of the presence of an intrinsic diode between the source and drain that could provide an undesirable discharge path to the channels during a charge sharing phase. Charge sharing is implemented only between adjacent channels.
Ideally, implementations of a charge sharing function in a scan driver device for a multichannel LCD panel should be possible even if the fabrication process technology does not contemplate the possibility of integrating relatively high voltage diodes, and yet support high voltage operated LCD panels (GOA panels). Moreover, for enhanced flexibility of use, it should be possible to share part of the channel activation charge among any couple of channels to be sequentially activated, not necessarily adjacent, and to support the use of an external capacitor as an ancillary charge storage element.